H.E.S.S.

High Energy Stereoscopic System

Gamma rays from the composite supernova remnant Kes 75

October 2008

Deep Chandra X-ray image of the supernova remnant Kes 75. The
explosion of a massive star created the supernova remnant, along
with a pulsar, a rapidly spinning neutron star. The low energy
X-rays are colored red in this image and the high energy X-rays
are colored blue. The pulsar PSR J1846-0258 is the bright spot
near the center of the image. In its rapidly rotating magnetic
fields, the pulsar generates a wind of highly relativistic
electrons and positrons, forming the pulsar wind nebula visible
as the blue emission region of high-energy X-rays. The nebula is
surrounded by a 3' shell emitting lower-energy X-rays. From
NASA/CXC/GSFC/F.P.Gavriil et al.

Kes 75, also known as
G29.7-0.3,
located in the constellation Aquila, is a prototypical young composite
supernova remnant, consisting of the pulsar PSR J1846-0258 created in
the supernova explosion, a nebula of high-energy electrons surrounding
the pulsar, and a 3' shell emitting mostly thermal X-rays (top image)
and radio (Fig. 1). With an age of about 700
years and an energy loss of close to 1037 ergs/s, PSR
J1846-0258 is one of the youngest and most energetic pulsars known in
the Milky Way
(Gotthelf
et al. 2000).
Chandra observations show burst-like variations in the X-ray
emission from the pulsar. The distance of Kes 75 was initially
estimated to about 19 kpc
(Becker
and Helfand 1984), causing concerns because of the implausibly
high X-ray luminosity, but recent determinations favor a closer
distance of 6 kpc
(Leahy and
Tian 2008). Kes 75 appears embedded in a region of relative high
gas density
(Morton et
al. 2007).

Its age, the large spin-down power of the pulsar and the dense gas
environment make Kes 75 an interesting target for observations in the
TeV range, probing the physics of young pulsar wind nebulae as well as
cosmic ray acceleration in supernova remnant shells, where the high
gas density provides a target for interactions of accelerated nuclei
and for production of gamma rays. Indeed, Kes 75 was discovered in the
HESS
Galactic Plane Survey as a source of very high energy gamma rays
(Fig. 2), first presented at the 2007 ICRC
(A. Djannati-Atai et
al. 2007). The latest data set corresponds to 30 h of live
observation time, slightly more than presented at the ICRC, and yields
a detection with more than 10 sigma significance. The gamma-ray source
(Fig. 3) is centered on the remnant and appears
point-like; however, a contribution from the 1.5' radius shell cannot
be excluded, given that the visible source size is determined by the
point spread function of the H.E.S.S. instrument of about 5'-6'. The
flux is at the level of about 2% of that of the Crab Nebula. As
mechanisms for gamma-ray production, both upscattering of ambient
photons by electrons of the pulsar wind nebula or interactions of
protons accelerated in the shell are possible; the low gamma-ray
luminosity implies, however, that cosmic-ray acceleration in the
remnant is not too efficient, with not more than about 10% of the
explosion energy converted into high-energy cosmic rays.

Fig. 3:
Gamma-ray map, smoothed with a Gaussian of width 0.05 degr.,
centered on the Kes 75 SNR position (indicated by the black
triangle). Significance contours at 8, 9 and 10 sigma are
overlayed in white. (Preliminary)